This invention relates to a traveling wave deflection system for use in a cathode ray tube (CRT), and more particularly to such a system having a pair of deflectors each having a helical conductor coiled around an elongate core of electrically insulating material. Still more particularly, the invention pertains to the optimization of the dispersion characteristic of the helical wave paths.
The electron beam in a CRT will be deflected in proportion with the deflection voltage if that voltage remains unchanged during the passage of the electrons through the deflection field. In case the deflection frequency is high, however, the deflection voltage may change during the passage of the electrons through the deflection system, making it difficult to deflect the beam as required.
A familiar solution to the foregoing problem is the traveling wave deflection system, for use in particular in CRTs for observation of signals from zero to several hundred megahertz in frequency. The traveling wave deflection system is such that the phase velocity of the deflection signal traveling through a pair of deflectors of helical or other configurations is made approximately the same as the speed of the electron beam. The deflection signal is thus made to act on the electron beam for a longer, sufficient period of time for its desired deflection, making possible the provision of a wide band CRT.
Japanese Pat. Pub. No. 57-10539 teaches a traveling wave deflection system in which each deflector has a grounded conductor, or a pair of such conductors, inserted between an insulating core and a helical conductor wound thereon. The grounded conductor or conductors are intended primarily to make the characteristic impedance of the traveling wave conductor constant in the traveling direction of the undeflected beam. Tobari et al. U.S. Pat. No. 5,038,075 suggests an analogous deflection system wherein the grounded conductors are so made as to compensate for an inductance drop toward the end of each traveling wave conductor and so to make the characteristic impedance thereof constant all along the beam path.
Such prior art systems have proved to possess a weakness, however, in that they are devoted solely to making constant the characteristic impedance of the traveling wave conductors, paying no attention to their dispersion characteristic (i.e. variation in speed of the traveling wave through the conductors). The provision of a wide band traveling wave deflection system requires not only the solution of the problem of the reflections of the deflection signal waveforms due to inconstancy of the characteristic impedance of the traveling wave conductors but also the improvement of their dispersion characteristic for faithful transmission of the deflection signal waveforms. Also required is the reduction of the waveform distortion resulting from the mismatching of the speed of the electron beam and the phase velocity due to the dispersion characteristic of the traveling wave paths.
The phase velocity of a wave will be constant regardless of frequency if it is traveling through a path that is not dispersive. Phase velocity in this case is expressed as EQU u=.omega./.beta. (1)
where
u=phase velocity, PA1 .omega.=angular frequency, PA1 .beta.=phase constant.
The nature of the transmission path is represented by the phase constant .beta.. The phase constant for a transmission path where inductance and capacitance per unit length are expressed as L and C is given by EQU .beta.=[1/(LC).sup.1/2 ].omega..
The phase velocity of a wave is a function of angular frequency if it is traveling through a dispersive path. Analyses of traveling wave deflectors indicate that the transmission of a pulse waveform without phase distortion requires constant phase velocity regardless of frequency and a linear phase characteristic, as indicated by Equation (1). A transmission path whose nature is expressible by Equation (1) is capable of distortionless transmission of signal waveforms, with a constant phase velocity regardless of frequency.
Graphically represented in FIG. 10 of the drawings attached hereto are the results of simulation experiments, showing a waveform A in response to the transmission of an input pulse Po through a transmission path in which EQU .beta.=.omega./c, and EQU u=c
where c is a constant. Also given in FIG. 10 is a waveform B in response to the transmission of the input pulse Po through a transmission path in which the phase constant is expressed as EQU .beta.=.omega./(c+a.omega.+b.omega..sup.2),
where a, b and c are all constants, and in which the phase velocity is a function of the angular velocity: EQU u=c+a.omega.+b.omega..sup.2.
The response waveform A in FIG. 10 demonstrates that distortionless transmission is possible if phase velocity is constant over angular frequency. On the other hand, in the case of a transmission path in which phase velocity increases with angular frequency, the response waveform B has a preshoot distortion and is slow in rise time. CRTs incorporating such traveling wave deflectors are inconveniently narrow in frequency band.
In traveling wave deflection systems of CRTs, the phase velocity of the input signal must be reduced to approximately one tenth of the speed of light in order to match the electron beam speed. This requirement has been met by use of helical conductors as guided signal paths, as in the prior art systems set forth previously. The pitch of the helices may be made one tenth of the length of each turn in order to approximate the required phase velocity.
However, despite their undisputable advantages, the helical conductors of the prior art deflection systems have proved still unsatisfactory for the provision of wide band CRTs. The neighboring turns of the helical conductors are, unavoidably, electrically coupled together. Such couplings are negligible at lower frequencies because then little or no potential differences are created between the conductor turns.
At higher frequencies, however, potential differences and therefore field couplings are created between the conductor turns, to such an extent that the capacitances between them become inconveniently high. Such capacitances have conventionally made the phase velocity increasingly higher with frequency, resulting in distortions of pulse waveforms such as that indicated at B in FIG. 10 and in limitations of the frequency band. Additionally, the phase velocity of the input signal has failed to match the electron beam speed, and band limitations have occurred by the effect of electron travel.